The present invention relates generally to refrigeration systems. More particularly, the present invention relates to transcritical refrigeration systems configured to improve temperature pulldown after system start-up.
In a typical refrigeration system that utilizes a circulating refrigerant, the refrigerant is circulated throughout a particular refrigerated area to remove heat from that area. The refrigerant enters the evaporator as a liquid or as a saturated mix of liquid and vapor and the liquid is evaporated (i.e., it boils off to pure vapor) as it absorbs heat from the refrigerated area. This process takes place at a refrigerant temperature somewhat below the temperature of the refrigerated area in order to facilitate heat transfer from the area to the refrigerant. The flow of refrigerant through the evaporator is normally regulated to maintain the temperature of the vapor exiting the evaporator at some fixed margin, or “superheat,” above the saturated temperature of the liquid-vapor mix. This assures that exactly enough refrigerant is circulated to match the heat load of the refrigerated area. Because the refrigerated area may not require constant cooling, the refrigeration system may be turned off for a period of time, thereby allowing the refrigerated area and the refrigerant to warm to a temperature at or near the ambient temperature. When the refrigerated area once again requires cooling, the refrigeration system is turned on, and the refrigerant will initially go through the process of evaporation at a temperature somewhat below the ambient temperature. As the refrigerated area is cooled, the temperature of the evaporating refrigerant will drop accordingly until the refrigerated area reaches the desired temperature and the system stabilizes again. The process of cooling a refrigerated area from a warmer temperature following a system shutdown to a desired cooler setpoint temperature is known as “pulldown.”
Refrigerants containing chlorine have been phased out in most of the world due to their ozone destroying potential. Hydrofluorocarbons (HFCs) have been used as replacement refrigerants, but these refrigerants also have high global warming potential. “Natural” refrigerants, such as carbon dioxide, have recently been proposed as replacement fluids. Unfortunately, there are problems with the use of these natural refrigerants as well. In particular, carbon dioxide has a low critical temperature, which causes the evaporator temperature and pressure to be above the critical point and in the supercritical region during start-up of the refrigeration system. When the refrigerant is at a temperature above the critical temperature, there are no separate liquid and vapor phases and so the normal process of evaporation cannot take place. When the evaporator temperature is supercritical there is no such thing as “superheat,” and therefore, the flow regulating device is unable to operate properly. As a result, it becomes very difficult to control the initial pulldown process that is necessary to bring the refrigerated area to the desired setpoint temperature and to return the refrigerant to a normal subcritical process.
Thus, there exists a need for a refrigeration system with improved pulldown control when a transcritical refrigerant, such as carbon dioxide, is used in a transcritical mode to provide cooling.